The present invention relates to a method and apparatus for the drying section of a paper machine. More particularly, the invention relates to a method used in the press or drying section of a paper machine to prevent the formation of positive and negative pressures, detrimental to the support contact between the web and the fabric, at pits or nips defined by the fabric or web and the surface of the cylinder.
The invention also relates to apparatus for carrying out the method of the invention, which includes a blow or blowing box in operative proximity with the nip and extending substantially over the entire width of the web. The blow box is connected to apparatus producing blowing air and is provided with one or a plurality of nozzle slots.
It is well known that a thin layer of air follows all moving surfaces. No gliding occurs between the air and a moving surface, but particles of air which contact the moving surface travel at the same speed as said surface.
The pit defined by the fabric such as, for example, felt, or wire screen, and the surface of the cylinder or roller is called the inlet nip when said fabric enters said pit, and the outlet nip when the fabric leaves said pit.
If all the limiting surfaces of the nip are impenetrable to air, flows of air are produced between the limiting layers both at the inlet nip and at the outlet nip, the directions of flow being opposite in relation to said layers. Thus, due to the damming of the flows at the limiting layers, there is a positive pressure at the inlet nip across the fabric, and, due to the suction effect of the flows at said limiting layers, there is a negative pressure at the outlet nip across the fabric.
When fabrics penetrable to air such as, for example, wire screens, are used in a previously known manner, the pressure differences produced by the flows at the limiting layers across the wire screen produce flows of air through said wire screen.
Several prior art pocket ventilation apparatus of paper machines are based on the pumping effect of open drying screens.
As is well known, the first and the second drying groups of a paper machine are usually provided with a so-called single-fabric guide system, which is frequently accomplished so that at the upper cylinders the paper web is between the fabric and the cylinder, and at the lower cylinders, said web is on said fabric.
The single-fabric guide system has several problems resulting from pressure differences on different sides of the fabric, caused by the flows at the limiting layers. The air tends to flow from a higher pressure area to a lower pressure area through the wire screen, and thereby to interrupt the support contact between the fabric and the paper web. The problematic points are the outlet nip and the inlet nip. At both points, the pressure difference across the wire screen tends to detach the paper web from the wire screen. After the paper web and the wire screen are detached, the web begins to behave in an unstable manner, due to flows of air occurring in the pocket. The detached web is frequently extended at the lower cylinders due to the effect of centrifugal force. The consequences are serious wrinkles at the inlet nip and, in the worst case, breakage of the web.
In order to solve the aforementioned problems, various seal constructions have been developed, whose objective is to prevent the entrance of the boundary-layer flows following the moving surface into the nip. Patent application No. DE-OS 2,712,184, for example, discloses a sealing construction for this purpose.
Finnish patent application No. 771056 discloses a method for the drying section of a paper machine for passing the web in a sealed guide system, in which method, a drying fabric or screen is used in the initial part of the drying section, especially in its first group of drying cylinders. The drying fabric or screen is positioned or arranged in a manner whereby the cylinders in one row are outside the loop of the fabric and the other cylinders are inside said loop, so that the web runs, making bends from one cylinder row to the other, all the time, as supported by the same fabric from the beginning to the end of the group of drying cylinders provided with that fabric. In this application, it is considered a novelty that a pressure difference is applied to the web at least at some of the cylinders of the row within whose sector the web is outside the fabric. The pressure difference is applied to the web via the hollow surfaces of the cylinders, so that the pressure prevailing outside the web is greater than the pressure prevailing in the cavities of the hollow surface. The purpose of this is to especially prevent the web from being detached from the fabric and to insure constant operation of the paper machine.
Finnish Pat. No. 54,954 discloses a method for insuring the transfer of the web from the press section to the drying section. In the method of the Finnish patent, the principal novelty is that a separate guiding drying cylinder, actually not belonging to such group, is provided between the group of drying cylinders and the press section. In order to keep the web in contact with the lower drying belt when said lower drying belt circulates around the upper cylinders of the particular cylinder group, an upper drying belt, of a width substantially equal to the width of the web, is passed onto the web in order to press the web against the lower drying belt within a sector substantially smaller than the sector within which the lower drying belt supporting the web covers the upper cylinders. The upper drying belt may be brought into contact with the guiding drying cylinder by an adjustably positioned guiding roller, at least at the stage in which the web is transferred from the press section to the drying section when the paper machine is being started. PG,6
Furthermore, with regard to the state of technology related to the invention, reference is made to Finnish patent application No. 793643 of the inventor, which patent discloses apparatus in the drying section of a paper machine for passing the web in a sealed guide system. A drying fabric of the apparatus is arranged or positioned in a manner whereby one or some of the cylinders or rollers of the drying section are placed outside the loop of the drying fabric and the other cylinder or roller, or cylinders or rollers, are placed inside said loop, so that the web passes from one cylinder or roller to another, supported by the fabric throughout the distance between cylinders or rollers. The apparatus comprises a suction box or suction boxes fitted to act on the drying fabric. In this application, it is considered a novelty that the suction box is, or the suction boxes are fitted to extend substantially over the entire length of the common run of the web and the drying fabric from one cylinder or roller to another. The suction of the suction box extends into the wedge space between the drying fabric and the drying cylinder or roller mantle, at least at the inlet side of the drying fabric and the web.
The prior art mechanical seals are limited, since they cannot be brought close enough to the moving web or fabric, due to the risk of damage to the web and wear of the fabric. It is impossible to seal an outlet nip by mechanical means, for example, since the moving paper web requires a distance of at least 10 mm from the seal. At an inlet nip, it is theoretically possible to place the seal closer to the fabric, but the problem point is then shifted to ahead of the seal, where a positive pressure caused by the dam pressure of the flow at the limiting layer is formed.
The efficiency of mechanical seals is also reduced with time, if the seals are placed so close to the moving surfaces that abrasion occurs.